MicroPeak — tiny peak-recording altimeter now available

MicroPeak is a miniature peak-recording altimeter. About the same size
and weight as a US dime (with battery), MicroPeak offers fabulous
accuracy (20cm or 8in at sea level) and wide range (up to 31km or
101k').

The size of the board was predicated with the premise that we
needed a battery included to avoid having wiring running between the
altimeter and the board, we found some small lithium coin-cell battery
holders for the CR1025 battery. These battery holders are rated to
hold the battery secure up to 150gs.

We'd already started playing with the
Measurement Specialties MS5607 pressure sensor
which offers amazing accuracy while using very little power. Taking full-precision
measurements every 96ms consumes about .2mA on average. Once on the
ground, we stop taking measurements entirely, dropping the power use
to around 1µA. It's also pretty small, measuring only 5mm x 3mm.

For a CPU, this little project didn't need much. The 8-bit
ATtiny85 comes in a
20qwfn package which is only 4mm x 4mm. When run at full speed (8MHz),
it consumes a couple of mA of power. Reduce the clock to a pokey
250kHz and the CPU has enough CPU power to track altitude while
consuming less than .2mA on average.

To avoid losing the battery, we wanted to avoid having it removed
while the board wasn't in use. So, we added a little power switch to
the board. The one we found is good to at least 50g.

Finally, we wanted to find a nice bright LED to show the state of the
device and to blink out the final altitude. The
OSRAM LO T67K
are bright-orange surface-mount LEDs that run happily on 2mA.

We used OSHPark.com to create prototype circuit
boards for this project. Because of the small size of the board, each
prototype run cost only $2 for three boards. It takes a couple of
weeks to get boards, but it's really hard to beat the price.